Various techniques have been employed in the past for the polymerization of polymers and copolymers of olefins. One of the approaches has involved employing catalysts based upon transition metal compounds such as titanium. Another approach has involved the employment of catalysts containing chromium. As a general rule these two types of catalysts produce polyolefins having somewhat different physical characteristics. For some applications, it is desirable to have polyolefins which have a blend of the properties that are produced by the titanium and the chromium catalyst. An example of such polyolefin compositions include those which have been referred to as having bimodal molecular weight distributions. Some techniques for preparing such bimodal polymers have involved the use of multiple reactor arrangements, sometimes loops, sometimes stirred tanks in which different polymerization conditions such as temperature, hydrogen, or comonomer are employed in the different reaction zones. Such multiple reactor schemes, while offering versatility in resin characteristics, can be less efficient than would be desired. The control of the multiple reactor schemes is difficult and reactor size for one resin may by necessity be miss-sized for another. Another technique which has been used to make bimodal polyolefin compositions involves merely blending polyolefin resins having different characteristics; for example, in an extruder to obtain a reasonable homogeneous mixture.
In addition, some attempts have been made to combine titanium and chromium on a single catalyst. Examples of such catalyst systems are disclosed in U.S. Pat. No. 3,622,521 and U.S. Pat. No. 4,041,224, the disclosures of which are incorporated herein by reference.
In addition, U.S. Pat. No. 4,285,834 discloses carrying out the polymerization of olefins using a mixture of two independent supported catalysts, one of which contains titanium or vanadium and the other of which contains chromium. While the use of these catalyst mixtures produces some interesting polymers, there is still room for improvement. The improved properties of most bimodal polymers such as those produced by blending result not just from the broad characteristics of the bimodal molecular weight distribution. Instead it is believed that to obtain polymers having the most desirable combination of properties for many applications, it is desirable to obtain a polymer in which the branching is concentrated mainly in the high molecular weight portion of the molecular weight distribution.
An object of the present invention is to provide novel catalyst mixtures.
Another object of the present invention is to provide a process for the polymerization of olefins to produce polymers having a broad molecular weight distribution with the branching concentrated in the high molecular weight portion of the molecular weight distribution.
Another object of the present invention is to provide a mixed catalyst system which can be employed effectively in particle form polymerization of olefins.
Other aspects, objects and advantages of the present invention will become apparent to those skilled in the art having the benefit of this disclosure.